Delivery of laser light through optical fibers or waveguide is an attractive way to direct laser energy to a point of interest in an eye safe fashion. Related applications of ultrashort lasers include, but are not limited to, laser surgery and multiphoton microscopy.
Fiber delivered ultrashort pulse light sources in the near IR (infrared) have many applications in medical imaging and multi-photon microscopy. For medical applications such sources need to be highly robust, have long term stability, and also comprise a minimal component count with a high degree of optical integration. Solid state and fiber laser based near IR light sources are commonly used. For applications in a clinical environment the robustness of fiber laser based ultrashort pulse sources is generally preferred, however.
Regarding two-photon microscopy, currently available systems are based on a certain set of fluorophores, which are typically excited at wavelengths near 800 nm, 920 nm, 1050 nm, 1300 nm and 1700 nm. Wavelengths of 1050 nm and 800 nm can for example be obtained from ytterbium (Yb) fiber lasers or frequency doubled erbium (Er) fiber lasers respectively. Wavelengths of 1700 nm can be obtained from Raman shifting of Er fiber lasers in optical fibers. However, no viable commercial solution for the generation of high power femtosecond pulses in the 920 and 1300 nm wavelength windows has yet been demonstrated, particularly when combined with fiber delivery.
For applications in two-photon confocal microcopy, in the absence of damage and photo-bleaching constraints, it is useful to maximize the number M of photons excited by two photon excitation. As is well known M can be shown to be proportional to M=const×P2/fτ, where P is the average laser power, f is the repetition frequency of the pulse train and τ is the pulse width. The typical average power required for two photon microscopy is in the range of 100 mW-1 W for pulse sources with repetition rates of around 100 MHz.